In crane boom assemblies, particularly those having extensible booms, it has become customary to provide an extension which is adapted to be mounted on the outer end of the boom to provide additional length thereby increasing the versatility of the crane. The extension is supported on the end of the boom by cables and can be held at different angular positions by using different cables or cable extensions. An example of an auxiliary or extension boom is disclosed in Forsythe et al. U.S. Pat No. 1,920,370. The extension disclosed in the Forsythe et al. patent is the type which must be manually removed from the boom when the extension is not required. This type of arrangement normally requires a separate vehicle for transporting the extension and also is time consuming to erect and remove from the crane boom.
Thus, proposals have been made for supporting the extension directly on the boom when the extension is not needed. Examples of this type of extension are disclosed in Grove U.S. Pat. No. 3,366,250; Lamer et al. U.S. Pat. No. 3,698,569; and Lamer U.S. Pat. No. 3,732,988.
In order to further increase the versatility of cranes of this type, it has more recently been proposed to adapt the unit so that a fly section may be attached to the outer end of the boom and may either be positioned in end-to-end relation with the boom and/or at various offset angles with respect to the main boom. In addition, additional length for the boom is provided by attaching a jib section on the outer end of the fly section.
Present known designs of fly section and jib section combinations allow the jib to be stored under the fly section when not in use. In order to rotate the jib section to a stored position on the fly section, it had been necessary to mount the jib section below the center line of the fly section. This means that when the jib section is moved to a usable position the center line of the jib section is offset from the center line of the fly section which results in undesirable bending stresses in the fly section when loads are placed on the jib section.
As indicated above, the jib section is normally adapted to be attached to the outer end of the fly section at a plurality of angular positions while the fly section is adapted to be attached to the boom in a plurality of angular positions to further increase the versatility of the entire unit. It has been determined that allowing the fly section to be angularly offset with respect to the boom section when the jib section is in a usable position will cause excessive bendng stresses on the main boom. This may also present a safety hazard since the end of the fly section is offset from the main boom sufficiently and may result in tipping of the crane when loads are supported thereon.